The present invention generally relates to an electrophotographic copying machine having a capability of successively producing a plurality of identical copies from one and the same electrostatic latent image formed on a photoreceptor carrier.
In an electrophotographic copying process, it is generally known that, at the time a copying paper is forcibly separated from the photoreceptor carrier after the transfer of a toner image from the electrostatic latent image on the photoreceptor carrier onto the copying paper, a pattern of charge is formed on the photoreceptor drum as a result of the occurrence of electrostatic discharge from a transfer means through the copying paper. For the description herein set forth this type of electrostatic discharge is referred to as "separation discharge".
The occurrence of the separation discharge will be discussed in detail with reference to FIG. 10. As shown therein, at the time the toner image formed on a photoconductive medium 1 is to be transferred onto the copying paper 2, charge opposite in polarity to the toner particles is applied by a DC transfer charger 3 to the copying paper 2. When the electrostatic force of the copying paper tending to attract the toner particles overcomes the electrostatic force of the photoconductive medium 1 retaining the toner particles in the form of the toner image, the toner image can be transferred onto the copying paper 2. It is to be noted that, in FIG. 10, the electrostatic charge of the toner particles is assumed to be of positive polarity, the corona discharge produced by the DC transfer charger 3 is assumed to be of negative polarity, and the photoconductive medium 1 in the form of a rotary drum and the copying paper 2 are assumed to be moved in the respective directions shown by the arrows a and b.
The copying paper 2 is separated from the photoconductive medium 1 immediately after the transfer of the toner image onto the copying paper 2. At this time, an electrostatic force of attraction acts between the toner image and the copying paper by a high charge imparted by the DC transfer charger 3 and, in most cases, it is greater than a natural force of separation relying on the stiffness of the copying paper 2. According to the prior art, a forced separating means of, for example, a belt type or a claw type, and/or an AC charge eraser 4 operable to apply an AC corona discharge to the copying paper 2 to neutralize the charge built on the copying paper 2 have been employed.
As the copying paper 2 separates from the photoconductive medium 1, a gap formed therebetween progressively increases accompanied by an increase in potential difference therebetween. The separation discharge is attributable to the increased potential difference, and the result thereof is a disturbance of the electrostatic latent image formed on the photoconductive medium. In the worst case, the disturbed latent image brings about a disturbance of an image area, and even though it may not occur, a background area tends to be disturbed because the potential at the background area is lower than that at the image area to be susceptible to discharge.
In an electrophotographic copying machine wherein one electrostatic latent image is used for one copy to be made, the latent image is formed each time the toner image has been transferred onto a copying paper. In such machine, the disturbance of the electrostatic latent image such as hereinabove discussed need not be taken into consideration. However, in the case of a similar copying machine having a capability of operating under a "retention copy mode" wherein one and the same electrostatic latent image is used for the successive production of a plurality of identical copies, the disturbance of the once-formed electrostatic latent image poses a serious problem in that the copies, except for the first one, will bear a distorted, unclear, and/or spoiled reproduced image.